Conventional air quench extrusion apparatus of `down-the-stack` type for spinning polypropylene yarn is several stories high and usually requires to be housed in at least a three story building. The extruder is located on an upper floor, the air quench cabinets are located on the next lower floor and may be six or seven feet in height, and below the air quench cabinets are long inter-floor tubes which extend down to a third or fourth floor. Below each inter-floor tube, which may be eighteen feet long, denier control rollers are positioned for pulling the yarn down through the `stack` with the denier of the undrawn yarn being determined by the drawn-down of the filaments in the quenching zone.
Disadvantages of this type of apparatus are its height, so requiring a special building, the complication of having operators on different floors, and the cost to build and install it.
One story air quench extrusion apparatus for producing polypropylene yarn are also known, but these have been found to have the disadvantage that they tend to produce less uniform yarn. There tend to be undesirable variations in denier from filament to filament and non-uniformily of denier along the length of a filament. Such yarn is usually cut up into staple fiber as it is not suitable, for example due to filament breaks during drawing, to be used as a textile quality continuous filament yarn.
It is surmised that to reduce the height, or length, or air quench extrusion apparatus for producing polypropylene yarn, the melt should be extruded at lower temperatures than in conventional `down-the-stack` apparatus, if the yarn is to be adequately and correctly cooled before it reaches the denier control rollers. It is also surmised that the use of lower temperatures normally contributes to denier irregularities and spinning breaks.